The present invention relates to a photosensitive composition which is particularly intended for the production of printing plates and is capable of yielding either a positive or a negative copy of an original, i.e., a positive copy can be produced either from a positive original or from a negative original.
In reproduction technique, supports of widely differing types which comprise various materials, and which are coated with photosensitive coatings, have been in general use for a long time in a very broad field which ranges from offset printing to the production of printed circuits. The photosensitive coatings are usually divided into two groups, i.e., negative-working coatings, by which a positive copy can be produced from a negative original, and positive-working coatings, by which a positive copy can be produced from a positive original.
The nature of negative-working coatings is such that the areas which have not been exposed to actinic light and have therefore remained unchanged are dissolved by a developer, as a result, a negative image is formed. In positive-working coatings, on the other hand, only those areas which have been exposed to actinic light are rendered soluble in an alkaline developer, and thus a positive image is formed.
For positive-working photosensitive coatings, o-naphthoquinone diazides are, at present, generally used. Material of this type are described, for example, in German Pat. Nos. 854,890; 865,109; 879,203; 894,959; 938,233; 1,124.,817; 1,109,521; 1,120,273; 1,118,606 and 1,114,705, and in U.S. Pat. No. 3,635,709. These o-naphthoquinone diazides make it possible to prepare positive-working photosensitive coatings which are of high stability and can easily be developed with aqueous-alkaline solutions, so that it is not necessary to employ developers on a basis of organic solvents.
The prior art has also disclosed photosensitive coatings based on o-quinone diazides, which undergo image reversal when they are heated for a particular period of time after exposure, such that a positive image is obtained from a negative original, and vice versa.
German Offenlegungeschrift No. 2,529,054 describes photosensitive coatings of the above-mentioned type. In these coatings, 1-hydroxyethyl-2-alkylimidazoline must be present, and a plate provided with such a coating must be heated at 105.degree. C. for at least 10 minutes. Photosensitive coatings of the above-indicated type are, moreover, described in German Offenlegungsschrift No. 2,855,723; the disclosed reversal process is effected by substances of the type ##STR1## but the plate must also be heated for at least 10 minutes, at temperatures above 100.degree. C. Published European Patent Application No. 24,916 discloses a composition which contains a photosensitive compound and, additionally, a resin belonging to the group of novolaks. This composition, which permits the production of both positive and negative copies from a positive original, must be heated for 30 minutes at 95.degree. C. or for 5 minutes at 150.degree. C. to effect the reversal process.
Published British Patent Application No. 2,082,339A describes a photosensitive coating for lithographic plates, which is based on o-quinone diazides and resol resins and which permits the formation of positive images from positive and also from negative originals. For the reversal process, this coating must be heated to at least 120.degree. C., if the treatment is to take no longer than 2 minutes.
U.S. Pat. No. 4,196,003 describes the use of o-napthoquinone diazides in the production of presensitized printing plates which can form positive or negative images, depending on the treatment to which the photosensitive coating is subjected. If the plate is exposed through a positive original and thereafter developed with an alkaline solution in the usual manner, such that the exposed areas are removed by dissolution, a positive image is obtained.
On the other hand, a positive image is formed if the plate is exposed through a negative original, heated for about 10 minutes at about 100.degree. C. after imagewise exposing, then subjected to an overall exposure using the same light source as before, and finally developed in the above-indicated alkaline developer. In the last-mentioned process, the action of heat causes a cross-linking reaction to occur in the imagewise exposed areas of the coating, by which the coating is rendered insoluble in the alkaline developer, whereas the remaining areas become soluble in the second exposure.
The photosensitive coating which is used to perform this reversal process comprises at least one o-naphthoquinone diazide, a phenol or cresol formaldehyde resin, and at least one secondary or tertiary amine. This amine acts as a catalyst in the thermal cross-linking process.
Amines are known to be toxic. The presence of an amine in the photosensitive coating constitutes a hazard to the health of persons who handle these materials and is also a potential source of contamination, since the temperatures to which the coating is subjected during the application and treatment thereof are so high that portions of the amines tend to evaporate into the surrounding atmosphere. Moreover, part of the coating and, consequently, also the amine contained therein, is dissolved in the developing procedure and it is, therefore, necessary to take particular measures in order to detoxify the spent developer.
The toxicity of the coating additives and the duration of the heat treatment are thus to be regarded as disadvantages of the prior art. Consumers wish to have materials requiring short treatment times.